1. Field of the Invention
The present invention relates to a grain-oriented electrical steel sheet, which has a film with excellent annealing resistance and does not include chromium therein, and a producing method therefor.
This application is a national stage application of International Application No. PCT/JP2009/056573, filed Mar. 30, 2009, which claims priority to Japanese Patent Application No. 2008-91051, filed Mar. 31, 2008, the content of which is incorporated herein by reference.
2. Description of Related Art
A grain-oriented electrical steel sheet has a crystal structure with (110) and [001] as a main orientation and is used much as a material of a magnetic iron core. In particular, a material that has low iron loss is required in order to reduce energy loss. With regard to such requirements, an iron alloy that includes iron and silicon is known such that magnetic domain refinement occurs and eddy current loss, which is a major factor in iron loss, is reduced when external tension is applied. In general, in order to reduce the iron loss of a grain-oriented electrical steel sheet that includes silicon of 5% or less, it is effective to apply tension to the steel sheet. The tension is typically applied by a film formed on the surface of the steel sheet.
Conventionally, a grain-oriented electrical steel sheet having a sheet thickness of 0.23 mm is subjected to a tension of about 1.0 kgf/mm2 by two layers of films with a first film that includes as a main compound forsterite formed by the reaction of oxides with an annealing separator on the surface of the steel sheet in a final annealing process and a second film that is formed by baking of a coating solution that includes as a main compound colloidal silica and phosphate. Such a film is required to have various functions, such as corrosion resistance, water resistance, sliding properties during machining, and annealing resistance during stain relief annealing, in addition to the effect of applying tension. Since a film merely composed of phosphate and silica had a problem, for example, in annealing resistance, the problem was solved by adding a chromium compound to the film conventionally as disclosed in Japanese Unexamined Patent Application, First Publication No. S48-39338. This is a method that chromic acid added to the coating solution removes harmful effect of the film component, which has an adverse effect on annealing resistance. Chromium itself is reduced into a trivalent state during baking, and may form a compound with phosphorus.
Although the method of forming a film that includes chromium is a technology that can realize excellent film characteristics by using trivalent chromium having a little environmental load, the current technology trend is to develop a tension film that does not use chromic acid, which needs attention in handling in its producing process. For example, Japanese Unexamined Patent Application, First Publication No. H6-65754 discloses a film that includes aluminum borate as a major compound. However, this has the problem of poor water resistance. In addition, Japanese Unexamined Patent Application, First Publication No. S61-235514 discloses a technology of TiN coating. However, this has the problem of cost, since it is a dry coating technology.
Meanwhile, Japanese Unexamined Patent Application, First Publication No. 2007-23329 discloses a technology that improves film characteristics, such as tension and water resistance, by adding a suspension liquid including Ti, Zr, or the like, or Fe into a coating solution in the forming of a film, and baking the coating solution on a steel sheet. This technology aims to improve characteristics by adding a metal compound in the colloidal state into the coating solution so that the metal compound stabilizes free phosphate as well as to improve a porous structure which is easily generated on the film that does not include chromium. Although this technology had a certain degree of effect in improving water resistance, the improvement effect was not always sufficient. In addition, due to the addition of the suspension liquid, neither the viscosity stability nor the aggregation resistance of the coating solution was sufficient and there was a possibility of problems in the stable formation of the film.
Japanese Unexamined Patent Application, First Publication No. 2005-187924 discloses a method of improving the characteristics of a film using sulfate of manganese or the like. However, in this technology, the film of the sulfate of manganese or the like is required to have a two-layer structure, with the upper layer being essentially a film composed of silica and an organic film. In addition, it was difficult to apply this technology to the current producing process of electric steel sheets. Therefore, this known technology was not able to sufficiently improve annealing resistance at a low cost.